Metals and alloys with desirable mechanical, magnetic, electronic, optical, or biological properties enjoy wide applications throughout many industries. Many physical and/or mechanical properties, such as strength, hardness, ductility, toughness, electrical resistance etc., depend on the internal morphological structure of the metal or alloy.
The internal structure of a metal or alloy is often referred to as its microstructure, although the micro-prefix is not intended here to limit the scale of the structure in any way. As used herein, the microstructure of an alloy is defined by the various phases, grains, grain boundaries and defects that make up the internal structure of the alloy, and their arrangement within the metal or alloy. There may be more than one phase, and grains and phases or phase domains may exhibit characteristic sizes that range from nanometers to, for example, millimeters. For single phase crystalline metals and alloys, one of the most important microstructural characteristics is grain size. For metals and alloys that exhibit multiple phases, their properties also depend on internal morphological properties, such as phase composition, phase domain sizes, and phase spatial arrangement or phase distribution. Therefore, it is of great practical interest to tailor the grain sizes of metals and alloys, across a wide range that spans from micrometers down to nanometers, as well as their phase compositions, phase domain sizes, and phase arrangements or phase distributions. However, in many cases, it is not understood exactly, or even generally, how a change in internal morphological properties, such as phase composition or microstructure will affect such physical properties. Thus, it is not sufficient simply to know how to tailor phase composition or microstructure.
It is very useful in characterizing a microstructure, to define a characteristic microstructural length scale. In the case of metals and alloys that are polycrystalline, the characteristic length scale as used herein refers to the average grain size. For microstructures containing subgrains (i.e. regions within a crystal that differ slightly in orientation to one another), the characteristic length scale as used herein can also refer to the subgrain size. Metals and alloys can also contain twin defects, which are formed when adjacent grains or subgrains are misoriented in a specific symmetric way. For such metals and alloys, the characteristic length scale as used herein can refer to the spacing between these twin defects. Metals and alloys can also contain many different phases, such as different types of crystalline phases (such as face-centered cubic, body-centered cubic, hexagonal close-packed, or specific ordered intermetallic structures), as well as amorphous and quasi-crystalline phases. For such metals and alloys, the characteristic length scale as used herein can refer to the average separation between the different phases, or the average characteristic size of each phase domain.
Additionally, there are many properties, such as optical luster, wettability with various liquids, coefficient of friction and corrosion resistance that depend on the surface morphologies of metals and alloys. Thus, the ability to tailor the surface morphologies of metals and alloys is also pertinent and valuable. However, in many cases it is not understood exactly, or even generally, how a change in surface morphology will affect these other properties. In general, as used herein, the term morphological properties may be used to refer to both surface morphology, and also to internal morphology.
There are many existing techniques that are capable of fabricating metals and alloys of different microstructures, including severe deformation processing methods, mechanical milling, novel recrystallization or crystallization pathways, vapor phase deposition, and electrochemical deposition (herein called electrodeposition).
However, many of these processing techniques have drawbacks. Some cannot provide a product of any desired shape, but rather are limited to relatively simple shapes such as sheets, rolls, plates, slugs, etc. Some cannot be used to make relatively large parts, without expending undue amounts of energy. Others provide some end product microstructures, but the control over such microstructures is relatively crude and imprecise, with only a few variables being changeable for a given process.
As a specific example of desirable properties, it is useful to provide alloy coatings on substrates. In many cases, it is beneficial that such coatings be relatively hard or strong, relatively ductile, and also relatively light per unit volume.
In other cases, it is beneficial to provide monolithic alloy pieces that are not connected to a substrate, or which have been removed from a substrate, as in the process of electroforming. In these cases, it is often beneficial that such pieces, or such electroforms, be relatively hard or strong, relatively ductile, and also relatively light per unit volume.
Steel has a characteristic strength to weight ratio, as do aluminum alloys, which are generally lighter than but not as strong as steel. Thus, it would be desirable to be able to produce an alloy that is as hard as steel, or nearly so, yet also as lightweight per unit volume as aluminum, or nearly so. Another, related desirable goal would be to produce an alloy that is harder than aluminum alloys, yet lighter, per unit volume, than steel.
The inventors hereof have determined that electrodeposition is particularly attractive because it exhibits the following advantages. Electrodeposition can be used to plate out metal on a conductive material of virtually any shape, to yield exceptional properties, such as enhanced corrosion and wear resistance. Electrodeposition can readily be scaled up into industrial scale operations because of relatively low energy requirements and electrodeposition offers more exact microstructure control since many processing variables (e.g. temperature, current density and bath composition) can be adjusted to affect some properties of the product. Electrodeposition can also be used to form coatings that are intended to remain atop a substrate, or electroformed parts that have some portions removed from the substrate onto which they were plated.
In addition to these advantages, electrodeposition also allows a wide range of metals and alloys to be fabricated by selection of an appropriate electrolyte. Many alloy systems, including copper-, iron-, cobalt-, gold-, silver-, palladium-, zinc-, chromium-, tin- and nickel-based alloys, can be electrodeposited in aqueous electrolytes, where water is used as the solvent. However, metals that exhibit far lower reduction potentials than water, such as aluminum and magnesium, cannot be electrodeposited in aqueous electrolytes with conventional methods. They can be electrodeposited in non-aqueous electrolytes, such as molten salts, toluene, ether, and ionic liquids. Typical variables that have been employed to control the structures of metals and alloys electrodeposited in non-aqueous electrolytes include current density, bath temperature and bath composition. However, with these variables, the range of microstructure that has been produced is limited. To date, no known method can produce a non-ferrous alloy that is as hard and ductile as steel, or nearly so, yet as light as aluminum, or nearly so, or, put another way, harder and more ductile than aluminum, yet lighter than steel.
Electrodeposition of nanocrystalline aluminum (Al) has been achieved from aluminum chloride based solutions by other researchers using direct current (DC), with additives, such as nicotinic acid, lanthanum chloride and benzoic acid While additives can effectively refine grain size, the range of grain sizes that can be obtained is limited; for instance, a very small amount of benzoic acid (0.02 mol/L) reduces the Al grain size to 20 nm and further increase in benzoic acid concentration does not cause further reduction in grain size. Additives can be organic, in the class known generally as grain refiners, and may also be called brighteners and levelers.
Electrodeposition of nanocrystalline Al has also been achieved by other researchers using a pulsed deposition current (on/off) without additives, but again, the range of grain sizes obtainable is narrow.
Processing temperature has also been found to affect the grain size of electrodeposited Al. However, using temperature to control grain size is less practical because of the long time and high energy consumption required to change the electrolyte temperature from one processing run to the next.
It would also be desirable to tailor mechanical, magnetic, electronic, optical or biological properties by manipulating parameters of the process that do not require changing electrolyte composition, such as by using additives that would not otherwise be necessary, or processing temperature, or other parameters that would be time or energy consuming to adjust, or energy intensive to use, or that would be difficult to monitor. By additives, it is meant generally grain refiners, brighteners and levelers, which include among other things nicotinic acid, lanthanum chloride, or benzoic acid, and organic grain refiners, brighteners and levelers.
It would also be desirable to be able to control such physical properties without necessarily understanding the relationship between microstructural or internal morphological characteristics such as grain size, phase domain size, phase composition and arrangement or distribution, and the physical and/or mechanical properties mentioned above. Similarly, it would be desirable to tailor surface morphology, or surface properties, such as optical luster, wettability by various liquids, coefficient of friction and corrosion resistance, by manipulating similarly convenient parameters, and further, without necessarily understanding the relationship between surface morphology and the surface properties mentioned above.
It would also be desirable to be able to create alloys, having a wide range of grain size, for instance from about 15 nm to about 2500 nm, and also to effectively control the grain size within this range. It would also be of great benefit to be able to use one single electrolytic composition, to sequentially electrodeposit alloys of different microstructures and surface morphologies. Finally, it would be of tremendous benefit to be able to provide a graded microstructure where one or all of the following are controlled through deposit thickness: grain size, chemical composition; phase composition; phase domain size; and phase arrangement or distribution.